Electrochemical reduction of (η 5-Cp)Fe(CO)3+ (1+) and (η5-indenyl)Fe(CO)3+(2+) in the presence of P- and As-donor nucleophiles (L) leads to rapid and efficient CO substitution by an electron-transfer-catalyzed (ETC) pathway to afford (η5-Cp)Fe(CO)2L+ and (η5-indenyl)Fe(CO)2L+. The CO substitution may also be effected quantitatively and rapidly by using trace amounts of chemical reducing agents such as NEt3 and Na/Pb. A detailed variable-temperature electrochemical study showed that the 19-electron radical 1 dissociates CO with a rate constant greater than 103 S-1 at -112 °C in butyronitrile. In contrast, 2 is relatively stable, with k-co being at least 106 times less than that for 1. Voltammetry with conventional electrodes and with microelectrodes under steady-state conditions allowed the mechanism of CO substitution in the 19-electron radicals 1 and 2 to be established as strictly dissociative. This fact, as well as the determination (from microelectrode steady-state experiments) that the rate of heterogeneous charge transfer for the process 2+→ 2 is fast while that for 2 →2- is slow, argues strongly that the indenyl ligand in 2 is η5-bonded and not η3-bonded as previously proposed. The results of extended Hückel MO calculations provide a simple explanation of the reduced reactivity of 2 in comparison to 1, without the necessity of invoking ring slippage. The LUMO's of 1+ and 2+ contain a large amount of metal character, as was confirmed by an examination of the ESR spectrum of 2. The LUMO's of 1+ and 2+ are both Fe–CO antibonding and have a similar amount of metal character but differ in that the LUMO of 2+ has a significant localization on the benzene ring of the indenyl ligand, with proportionately less localization on the CO ligands. Accordingly, the rate of dissociation of CO is much greater for 1 than for 2. The origin of this effect can be traced to the presence of a low-lying π* orbital in the indenyl anion that is predominantly localized on the benzene ring and which has the proper symmetry to interact with one of the two LUMO's on the Fe(CO)32+fragment. In effect, the indenyl ligand acts like an electron sink when 2+ is reduced, but this involves neither ring slippage to η3bonding nor a diminution of electron density on the metal in comparison to that in 1. The principal conclusion is that changing from cyclopentadienyl to indenyl ligands should greatly retard dissociative substitutions at 19e- centers (inverse “indenyl effect”), while, for similar reasons, substitutions should be accelerated at 18e- centers (A or D mechanism) and 17e- centers (A mechanism), in accordance with the well-known indenyl effect. Also reported in this study is the X-ray structure of [(η5-indenyl)Fe(CO)3]PF6: orthorhombic, space group Puma, a = 9.7911(8) Å, b = 7.5975(11) Å, c = 19.909(2) Å, Z = 4, 1405 unique reflections, R1 = 0.075, wR2 = 0.204.